Tensile deformation of B19‘ monoclinic martensite in nanocrystalline NiTi wires

An updated view of the deformation mechanism of B19‘ monoclinic martensite in NiTi wires deformed in tension until fracture will be presented based on the recent results of the: i) analysis of martensitic microstructures in deformed wires by TEM [1], ii) analysis of texture evolution during the tensile deformation [2], and iii) mechanics modelling of martensite deformation [3].

It will be shown that the reoriented martensite is (001) compound twinned [1,2] (polycrystal grains tend to be occupied by a single (001) compound twinned martensite variant). The state of the art view of the martensite reorientation as „detwinning“ is thus largely incorrect.

It will be claimed that plastic deformation of the B19’ martensite proceeds via a peculiar deformation mechanism involving [100]/(011) dislocation slip assisted deformation twinning and kinking [2,3]. This deformation mechanism involving rolling over (20-1) and (100) deformation twinning in later deformation stages enables plastic deformation of the NiTi wire at very high stress ~1 GPa up to very large strains ~50%. The plastically deformed martensite retransforms to the heavily dislocated austenitic microstructure containing {114} austenite twins on unloading and heating, accounting for large unrecovered strain and allowing for refinement of the austenitic microstructure of the wire down to nanolevel.